If it was accepted that David Bain did not kill his family that June 1994 morning, then who did? There seems only one other possibility - and one favoured at least during the earliest stages of the investigation - that Robin Bain murdered his wife Margaret, then his daughters Laniet and Arawa, then his son Stephen before shooting himself. A message was left on the computer, ostensibly from him for David saying, ‘sorry, you are the only one who deserved to stay.’
Theorists point to what they say are compelling facts to substantiate this scenario:

They say Laniet returned home that weekend to confront her parents with her father’s alleged incest with her over several years. Bain could have been looking at serious criminal charges if this had been true and had it been reported to the authorities, giving him a murder motive.

They say Robin's marriage to Margaret was essentially over, with Robin living in a schoolhouse at Tairei Beach during the week and a shabby caravan outside the family home at weekends.

Not all bloodstains on Robin's clothes were analysed before the trial and supporters say there is the possibility some could have come from other family members. If so, this would have severely damaged the prosecution case and strengthened the evidence against Robin.

The firearms discharge residue test, which indicates the presence of gunpowder on people who have recently fired a gun, was not carried out on Robin until five hours after the police arrived. This may have made the test inconclusive, as residue can disappear within hours of a shooting. In addition, instead of the tests being done at the scene, they were carried out at the mortuary, after the body had been moved without the hands being protected.

Robin had six recent injuries to his hands.

There were 20 empty cartridge shells in Robin’s caravan, indicating that Robin had access to, and had used David's rifle on previous occasions.

Note: Some of these issues were addressed and discounted by police investigating the original investigation.